Direct imine formation by oxidative coupling of alcohols and amines using supported manganese oxides under an air atmosphere

Article abstract of DOI:10.1039/c4gc00336e

Manganese oxides loaded on various supports have been prepared and studied for the direct imine formation by oxidative coupling of alcohols and amines. Among the catalysts, hydroxyapatite supported manganese oxides (MnO _x/HAP) show the best activity and selectivity for this reaction in the absence of an additional base using air as the environmentally benign terminal oxidant. NH₃-/CO₂-TPD results show that the amphoteric properties of MnO_x/HAP are crucial for this reaction to obtain a satisfactory yield. Various aromatic alcohols and amines are smoothly transformed into the corresponding imines in good to excellent yields. The catalyst is reusable and gives 98% yield of the product in all 9 reuse tests. Compared with the fresh catalyst, the XRD and SEM of the reactivated MnO _x/HAP after nine reactions do not show any obvious change. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00336e

Green Chemistry
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Direct imine formation by oxidative coupling of
alcohols and amines using supported manganese
oxides under an air atmosphere†
Cite this: Green Chem., 2014, 16,
3328
Bo Chen,a,b Jun Li,a Wen Dai,a,b Lianyue Wanga and Shuang Gao*a
Manganese oxides loaded on various supports have been prepared and studied for the direct imine for-
mation by oxidative coupling of alcohols and amines. Among the catalysts, hydroxyapatite supported
manganese oxides (MnOx/HAP) show the best activity and selectivity for this reaction in the absence of an
additional base using air as the environmentally benign terminal oxidant. NH3-/CO2-TPD results show
that the amphoteric properties of MnOx/HAP are crucial for this reaction to obtain a satisfactory yield.
Various aromatic alcohols and amines are smoothly transformed into the corresponding imines in good
to excellent yields. The catalyst is reusable and gives 98% yield of the product in all 9 reuse tests. Com-
pared with the fresh catalyst, the XRD and SEM of the reactivated MnOx/HAP after nine reactions do not
show any obvious change.
Received 24th February 2014,
Accepted 8th April 2014
DOI: 10.1039/c4gc00336e
www.rsc.org/greenchem
alcohols and amines under inert atmosphere protection is
highly atom economical,33–40 but active noble metal complexes
Introduction
Imines are key N-containing intermediates and are extensively and high reaction temperatures are employed in these reac-
used in the synthesis of organic and pharmaceutical com- tions. The direct coupling of alcohols and amines has also
pounds.1,2 Traditionally, the condensation of carbonyl com- been performed under oxidative conditions,41–57 which is a
pounds and amines is the most prevalent and convenient comparatively greener and more practical method. Moreover,
method for the preparation of imines,3 whereas this protocol water is the only byproduct. To date, relevant research has
usually requires Lewis acid-promoted,4–6 extremely active alde- mainly focused on supported noble metal heterogeneous
hydes and dehydrating agents.7 Over the past few decades, (Pd,41 Pt,42 Au,43–46 Ru47) catalysts. However, most of them
considerable efforts have been devoted to the direct synthesis employed pure oxygen as the oxidant and some required high
of imines by dimerization of primary amines,8–14 oxidation of temperatures, a large amount of base and excess alcohols or
secondary amines15–24 and other methodologies.25–31 However, amines. Only two non-noble metal reaction systems were
several drawbacks still remain, for example the condensation reported using
a
stoichiometric amount of the oxidant
48
of amines is not an efficient way to obtain unsymmetric MnO2
and
a catalytic amount of molecular sieves
imines14 and primary amines may also dehydrogenate into K-OMS-2,49 yet either elevated temperatures or excess amines
unwanted nitrile byproducts.32
were required to achieve a high yield. Homogeneous transition
An elegant alternative strategy to synthesize imines by metal (Fe,50 Cu,51–53 Pd55) catalysts have also been explored for
direct cross-coupling of alcohols and amines has been develo- this one-pot reaction under mild conditions, but these systems
ped, which has received much attention, and is a potentially suffer from the difficulty in recovery and reuse of the catalysts.
more environmental and general method to synthesize imines, Moreover, addition of base, ligands, and utilizing excess
because alcohols are readily reactive and green starting alcohols or amines often bring separation problems.
reagents, and diverse types of the desired imines can be
From substantial and green chemistry points of view,
obtained. Among them, the dehydrogenative coupling of finding a more economical and environmentally friendly pro-
cedure to accomplish this reaction will be attractive and desir-
able. Accordingly, our basic aim for the reaction design is to
aDalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian National
Laboratory for Clean Energy, DNL, Dalian, 116023, People’s Republic of China.
E-mail: sgao@dicp.ac.cn; Fax: +086-0411-84379728; Tel: +086-0411-84379728
employ (i) non-noble metal heterogeneous catalysts, (ii) equi-
valent of substrates without any promoter (e.g., base), and (iii)
air as the oxidant. To realize this goal, active manganese
oxides supported on hydroxyapatite (MnOx/HAP) were syn-
thesized by the deposition–precipitation method, which acted
as efficient catalysts for the oxidative coupling of alcohols and
bUniversity of Chinese Academy of Sciences, Beijing, 10049, People’s Republic of
China
†Electronic supplementary information (ESI) available: Experimental details,
1H NMR and 13C NMR spectra are provided. See DOI: 10.1039/c4gc00336e
3328 | Green Chem., 2014, 16, 3328–3334
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amines to obtain the desired imines under mild conditions.
To the best of our knowledge, this is the first time when sup-
ported non-noble metal catalyst is used for imine formation
from alcohols and amines under an air atmosphere.
Results and discussion
Catalyst optimization
In a preliminary experiment, a model imine formation from
benzyl alcohol 1a and aniline 2a (1 : 1 molar ratio) was per-
formed over a series of supported and commercial manganese
oxides. The results are shown in Table 1. Among the various
catalysts tested, MnOx/HAP showed a superb advantage over
others. The yield was up to 93%, and the analytically pure
form of compound 3a can be easily obtained by simple fil-
tration, concentration under reduced pressure, and finally
drying in a vacuum to remove volatile organic impurities
(Table 1, entry 1). No reaction was observed with only the
support (HAP) or in the absence of the catalyst (Table 1, entries
2–3). It was important to note that the manganese salt
Mn(OAc)2·4H2O itself did not catalyze the reaction and the
active species cannot be generated in situ (Table 1, entries 4–5).
MnOx/TiO2 similarly gave an impressive conversion of 1a (99%),
but only gave 77% yield after 24 h (Table 1, entry 6). Other poten-
tial catalysts such as MgO, Al2O3, and SBA-15 supported manga-
nese oxides were also tested and found to be less effective or
inactive towards 3a (Table 1, entries 7–9). Commercial MnO2
and Mn3O4 could only give the desired imine with the yields of
9% and 1%, respectively (Table 1, entries 10 and 11).
Fig. 1 CO2-TPD (a) and NH3-TPD (b) profiles of different catalysts.
inactive for this reaction, and the purely basic (MgO) and
acidic (Al2O3) supported manganese oxides showed moderate
yields. Interestingly, a relatively acidic support (TiO2) exhibited
high activity for the reaction, but gave a 77% yield of the imine
3a. Besides 3a, we also detected azobenzene and azoxybenzene
byproducts generated by the self-coupling of aniline 2a. There-
fore, without enough 2a to condense with the intermediate
aldehyde, the yield and selectivity of imine 3a decreased. The
as-synthesized HAP material with the amphoteric nature was
crucial for this reaction to obtain a satisfactory yield.44 After
loading manganese oxides, the basic and acidic sites further
enhanced. So without an additional base or excess of sub-
strates, MnOx/HAP showed the best activity and selectivity
towards the reaction.
To rationalize the support effects, the surface acidity and
basicity sites of the catalysts were measured by CO2- and
NH3-TPD (Fig. 1). Comparing the characterization with cataly-
tic results, the inert SBA-15 supported manganese oxides were
Table 1 Synthesis of benzylimine catalyzed by various catalystsa
After confirming MnOx/HAP as the most suitable catalyst,
we optimized the reaction conditions further. Catalysts with
different Mn loadings (2.5, 5, 10 wt%) were also tested. Inter-
estingly, it was found that 5 wt% MnOx/HAP gave the best
result, lower or higher Mn loadings both decreased the yield of
3a (Table 2, entries 1–3), which could be explained by that a
balanced environment of manganese oxides and the ampho-
teric sites of the HAP surface were important for the reaction
to obtain the desired product. The subsequent experiment
focused on the effects of the solvent; as previous literature
revealed,49 non-polar and aprotic toluene was proved to be the
most suitable solvent for this transformation, and choosing
MeCN, EtOH, THF, and DCE as the solvent, the yields of the
product were 38%, 20%, 41%, and 18%, respectively (Table 2,
entries 4–7). The temperature had a significant influence on
the reaction, and the yield of the product decreased dramati-
Entry
Catalyst
Conv.b (%)
Yieldb (%)
1
2
3
4
5
6
7
8
MnOx/HAP
HAP
None
Mn(OAc)2·4H2O
Mn(OAc)2·4H2O + HAP
MnOx/TiO2
MnOx/MgO
MnOx/Al2O3
MnOx/SBA-15
MnO2
99
<1
Trace
<1
<1
99
63
33
5
93 (92)c
<1
Trace
<1
<1
77
55
32
4
9
10
11
11
1
9
1
Mn3O4
a Reaction conditions: benzyl alcohol (1 mmol), aniline (1 mmol), cally as the temperature was decreased to 60 °C (Table 2,
catalyst (10 mol%), toluene (2 mL), air balloon, 80 °C, 24 h.
entry 8). When the reaction was performed under an N2 atmo-
sphere, only 15% yield was obtained, confirming that air as
confirmed by GC-MS; the conversion and yield are based on the
b Determined by GC using diphenyl as the internal standard and
alcohol to imine ratio. c Isolated yield.
the oxidant was essential for the reaction (Table 2, entry 9).
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Table 2 Optimization of the reaction conditionsa
which was subsequently nucleophile attacked by amines to
obtain imines. Notably, the rate constant k1 for the alcohol oxi-
dation was 3.91 × 10−5 s−1 and for condensation k2 was 1.07 ×
10−4 s−1 (Fig. S1†), indicating that the rate-determining step of
the overall reaction was the initial alcohol oxidation. Although
the active catalytic species of MnOx/HAP have not yet been fully
clarified, we attempted to identify the effects of the catalyst in
each step. In the condensation step, this procedure usually
occurred easily when it came to aldehydes and amines. With
careful operation, both HAP and MnOx/HAP were found to
obviously expedite the condensation of aldehydes and amines
in the initial 45 min (Fig. S2†), which can be attributed to the
benefit of Lewis acid sites of the catalyst.4–6,44 We also per-
formed the benzyl alcohol oxidation over MnOx/HAP under the
same conditions, and a quantitative yield of benzaldehyde was
obtained after 24 h (not shown). Thus, MnOx/HAP not only
played an important role in alcohol oxidation but also in the
acceleration of the consequent condensation step.
Entry
Catalyst
Solvent
Conv.b (%)
Yieldb (%)
1
2
3
4
5
6
7
8
9
2.5 wt% MnOx/HAP
5.0 wt% MnOx/HAP
10 wt% MnOx/HAP
5.0 wt% MnOx/HAP
5.0 wt% MnOx/HAP
5.0 wt% MnOx/HAP
5.0 wt% MnOx/HAP
5.0 wt% MnOx/HAPd
5.0 wt% MnOx/HAPe
Toluene
Toluene
MeCN
EtOH
THF
DCE
Toluene
Toluene
Toluene
82
99
96
56
22
43
18
44
16
77
93 (92)c
90
38
20
41
18
41
15
a Reaction conditions: benzyl alcohol (1 mmol), aniline (1 mmol),
catalyst (10 mol%), solvent (2 mL), air balloon, 80 °C, 24 h.
b Determined by GC using diphenyl as the internal standard and
confirmed by GC-MS; the conversion and yield are based on the
alcohol to imine ratio. c Isolated yield. d The reaction temperature was
60 °C. e N2 balloon.
Reaction scope
To further understand the reaction pathway in detail, we
conducted a kinetic experiment for the imine formation via
oxidative coupling of 1a and 2a with MnOx/HAP under the opti-
mized conditions (Table 1, entry 1) and the product of the GC
yield was detected by continuous sampling (Fig. 2). The desired
imine was the major product during the whole course of the
reaction, and the amount of aldehyde formed by the selective
oxidation of alcohol was always lower than 5%. Based on the
above experimental observations and previous reports, the poss-
ible reaction pathway is shown in Scheme 1. Alcohols were oxi-
dized to the corresponding carbonyl intermediate under air,
Having smoothly achieved the synthesis of imine 3a, we
extended the general applicability of this catalytic system.
Table 3 MnOx/HAP-catalyzed synthesis of imines from various
alcohols and anilinea,b
Fig. 2 Reaction profile for the formation of imine from 1a (1 mmol) and
2a (1 mmol) over MnOx/HAP (metal: 10 mol%) at 80 °C under air balloon.
a Standard reaction conditions: alcohol (1 mmol), aniline (1 mmol),
MnOx (10 mol%), toluene (2 mL), 80 °C, air balloon, 24 h.
b Determined by GC and confirmed by GC-MS; the yield is based on
the alcohol to imine ratio. Numbers in parentheses are conversion and
yield, respectively. c Isolated yield. d The main byproduct was the
intermediate aldehyde.
Scheme 1 Proposed reaction pathway of MnOx/HAP-catalyzed imine
formation from alcohols and amines.
3330 | Green Chem., 2014, 16, 3328–3334
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First, various primary alcohols were tested with aniline 2a as electron-donating and electron-withdrawing substituted anilines
the coupling partner (Table 3). Notably, both electron-rich were reacted efficiently with benzyl alcohol to afford the desired
(p-OMe-, and p-Me-) and electron-deficient (p-Cl-, p-Br-, p-F-, amines in good to excellent yields (Table 4, entries 1–2, 5, 8–9).
and p-NO2-) aromatic alcohols in our case can be transformed Commonly, meta- and ortho-substituted amines had lower con-
into the desired imines in excellent yields (Table 3, entries versions and yields than para-substituted amines, which could
1–3, 6, 9–11). The position of the substitution on the phenyl be explained by steric effects (Table 4, entries 2–7). Interestingly,
ring of alcohols affected the reaction to some extent, meta- and 1-napthylamine gave a moderate yield of 76% (Table 4,
ortho-substituted alcohols generally gave a slightly lower yield entry 10). The reactions of both cyclic secondary amines and
than para-substituted ones (Table 3, entries 3–8). Surprisingly, primary aliphatic amines gave even higher yields (Table 4,
1-naphthalene methanol with a more steric hindrance group entries 11–13). It was noteworthy that the reaction also pro-
unexpectedly worked well and obtained an excellent yield, ceeded successfully with benzylamine, p-Cl- or p-OMe-substi-
which might be ascribed to the profit from the rigid ring tuted benzylamine, which might be self-coupling to produce
(Table 3, entry 12). When less activating cinnamyl alcohol was symmetric imine byproducts (Table 4, entries 15–17).
tested, the yield decreased to 30% (Table 3, entry 13). And ali-
In view of both academic research and industrial appli-
phatic n-octyl alcohol failed to convert into the corresponding cations, an outstanding heterogeneous catalyst not only had
aliphatic imine (Table 3, entry 14). Furthermore, our catalyst efficient catalytic activity and selectivity, but also endowed
also tolerated with N-containing alcohol and gave a moderate with large-scale production and stable recyclability. MnOx/HAP
yield of the corresponding imine (Table 3, entry 15).
was tested in gram scale synthesis of N-hexyl-1-phenylmethani-
Next, the reactions between benzyl alcohol and different mine under the standard conditions, and the yield was up to
amines were tested (Table 4). Similarly, the electronic effect 99% (1.13 g) after 48 h (Scheme 2). The result suggests that the
had little impact on the reactivity of the reaction. A series of MnOx/HAP catalytic system was a highly practical application
for imine synthesis. The recycling data in Fig. 3 show that the
catalyst retained robust stability through simple regeneration
Table 4 MnOx/HAP-catalyzed synthesis of imines from various amines
and benzyl alcohola,b
by calcination at 500 °C for 4 h, and gave 98% yield of the
product in all 9 reuse tests for the oxidative coupling of benzyl
alcohol and hexylamine. The hot filtration test was carried out
and it was found that no further consumption of benzyl
alcohol took place after the catalyst was filtered off at the con-
version of 67% (Fig. S3†). Inductively coupled plasma (ICP)
analysis confirmed that the Mn concentration in the filtrate
was less than 0.1 ppm, which demonstrated that the catalysis
Scheme 2 Gram-scale synthesis of N-hexyl-1-phenylmethanimine
over MnOx/HAP.
a See Table 2 for standard reaction conditions. b Determined by GC
and confirmed by GC-MS; the yield is based on the alcohol to imine
ratio. Numbers in parentheses are conversion and yield, respectively.
c The main byproduct was the intermediate aldehyde. d Isolated yield.
Fig. 3 Recovery and reuse of the catalyst MnOx/HAP. Reaction con-
ditions: benzyl alcohol (1 mmol), hexylamine (1 mmol), MnOx (10 mol%),
toluene (2 mL), air balloon, 80 °C, 24 h, and the GC yield (%) based on
alcohol to imine.
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column) and GC calculations of yields were performed on an
Agilent 7890A (SE-30 capillary column, FID detector) using
N2 as a carrier gas. 13C NMR and 1H NMR spectra were
recorded on a Bruker DRX-400 spectrometer operating at
400 MHz, using CDCl3 as the solvent with tetramethylsilane as
an internal reference.
Preparation of catalysts
Hydroxyapatite (HAP, Ca10(PO4)6(OH)2, specific surface area:
37.9 m2 g−1) was prepared according to the literature58
through a liquid precipitation method and finally calcined at
500 °C for 3 h. MgO (107.4 m2 g−1) was prepared by precipi-
tation from magnesium nitrate in distilled water by adding
liquid ammonia; the pH of the solution was maintained at
10 under continuous stirring at room temperature for 12 h;
then, the solution was filtered and washed with distilled water,
dried at 120 °C, and finally calcined at 500 °C for 3 h. SBA-15
was synthesized following the reported procedure,59 and then
dried at 110 °C overnight. TiO2 and Al2O3 were purchased from
Sinopharm Chemical Reagent Co., Ltd and were calcined at
500 °C for 3 h before use.
MnOx/HAP (Mn loading = 5 wt%) was prepared by a depo-
sition–precipitation (DP) procedure. Briefly, 1.00 g of HAP was
dispersed in an aqueous solution of Mn(OAc)2·4H2O (50 mL,
9.1 × 10−3 M) under stirring, and then an aqueous NaHCO3
(1 M) solution was added dropwise into the mixture until the
pH was brought to 7.5. The mixture was continuously stirred
for 24 h, filtered and washed with distilled water. The obtained
solid was dried at 80 °C overnight and calcined at 500 °C for
4 h. SBA-15, Al2O3 and TiO2-supported MnOx (5 wt%) and
MnOx/HAP (2.5 and 10 wt%) were prepared in the same way as
the preparation of MnOx/HAP (5 wt%). MnOx/MgO was pre-
pared by a routine impregnation method. After stirring at
room temperature for 24 h, the solid was filtered and washed
with distilled water several times, dried at 80 °C overnight and
calcined at 500 °C for 4 h.
Fig. 4 SEM images of 5 wt% MnOx/HAP before (a) and after the ninth
run (b); XRD patterns of HAP and MnOx/HAP (c).
was truly heterogeneous in nature. Moreover, compared with
the fresh MnOx/HAP, the SEM and XRD images of the reacti-
vated MnOx/HAP after nine reactions did not show any obvious
change. The manganese oxides were finely dispersed at the
edge of the support and the crystallinity of the support was
retained (Fig. 4), which are in good agreement with their
excellent reactivity.
Conclusions
In summary, we have reported direct imine formation by oxi-
dative coupling of alcohols and amines using MnOx/HAP as
the first non-noble metal heterogeneous catalyst under an air
atmosphere. Without an additional base and excess of alcohols
or amines, diverse types of substituted imines can be obtained
in good to excellent yields. The catalyst can be easily recovered
without significant loss of catalytic activity, and the desired
imines can be obtained in an analytically pure form by simple
filtration, concentration and drying in a vacuum. Moreover,
this inexpensive and simple catalyst can afford the desired
product in excellent yields on a large-scale experiment. These
results provide an economical and practical way to obtain
useful imines.
Catalyst characterization
The crystal structures of HAP and MnOx/HAP (fresh and
reused) were characterized from power X-ray diffraction (XRD)
patterns on a Rigaku D/Max 2500PC diffractometer equipped
with a Cu Kα radiation source (λ = 1.5418 Å) at a scanning rate
of 0.05°/s (2θ from 10° to 70°). The Mn lodging of the catalyst
and the catalyst leaching were measured by inductively
coupled plasma atomic emission spectroscopy (ICP-AES) using
a Perkin-Elmer OPTIMA 3300DV. The detection limit was
0.10 ppm. Nitrogen-adsorption isotherms were obtained at
−196 °C using a QuadraSorb SI4 Station. Before the measure-
ment, the samples were degassed in a vacuum at 300 °C for
3 h, and the BET surface areas of the samples were calculated
using adsorption data. Temperature-programmed desorption
(TPD) experiments of NH3 and CO2 were carried out on a
Micromeritics AutoChem II 2910 analyzer. The samples
Experimental section
General
Commercially available chemicals were used without further (100 mg) placed in a quartz tube were pre-treated in a flow of
purification unless otherwise noted. Products were confirmed Ar (20 mL min−1) at 500 °C for 0.5 h, followed by cooling the
by a GC-MS (Agilent 7890A GC/5973 MS, SE-54 capillary sample to 50 °C under an Ar flow. NH3 or CO2 (1 mL each
3332 | Green Chem., 2014, 16, 3328–3334
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General procedure for catalytic reaction
Typically, a mixture of alcohol (1 mmol), amine (1 mmol),
catalyst (125 mg, 10 mol%) and toluene (2 mL) was added into
a 25 mL Schlenk tube and vigorously stirred at 80 °C under air
balloon for 24 h. The conversion and yields were analysed by
GC based on the ratio of alcohol to imine, and the product
mixtures were identified by GC-MS. Analytically pure form of
imines can be easily obtained through the following steps:
after the reaction completion (negligible alcohols, amines, and
aldehydes in solution) and cooling to room temperature, the
catalyst was separated by simple filtration and washed with
ethyl acetate several times; the filtrate was collected and con-
centrated under reduced pressure, and finally dried in a
vacuum to remove volatile organic impurities.
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Acknowledgements
This work was supported by the National Natural Science
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